Field of the Invention
The present invention relates to a cooling device for an internal combustion engine, and a failure diagnosis method for a cooling device for an internal combustion engine. More particularly, it relates to a cooling device for an internal combustion engine having a failure diagnosis function for a thermostat valve and a failure diagnosis method for the cooling device of an internal combustion engine.
Description of the Background Art
Japanese Patent Laying-Open No. 2007-056722 discloses a cooling device for an internal combustion engine, in which a cooling passage connecting an engine coolant water passage provided in the engine to a radiator is disposed, and an electric pump circulating coolant water in this cooling passage is provided. This cooling device includes a failure detection system which performs a failure diagnosis for a thermostat valve adjusting a temperature of coolant water by switching cooling passages.
In this failure detection system, a coolant water temperature detected or estimated by a coolant water temperature sensor is compared with a preliminarily set reference value to perform the failure diagnosis for the thermostat valve. At this time, when a flow rate of the coolant water is increased by driving of the electric pump, a heat transfer rate from the engine to the coolant water is changed, thus a correction coefficient for correcting the reference value is set larger as the coolant water flow rate is larger.
Even when the water temperature is provided which in nature does not allow the thermostat valve to be opened, if the water pressure in the coolant water passage is raised by driving of the pump, a leakage flow rate occurs in the cooling passage. The leakage flow rate represents a flow rate of the coolant water flowing to the radiator in a closed state of the thermostat valve. In this case, even though the thermostat valve is closed, engine-side coolant water in the engine coolant water passage is mixed with radiator-side coolant water in the cooling passage connected to the radiator, so that the temperatures of coolant water on both sides come close to likely cause lowering in the accuracy of the failure diagnosis.
More in detail, in the case where the thermostat valve is connected to a radiator circulation passage, which allows coolant water discharged from the engine coolant water passage to pass through the radiator and return to the engine coolant water passage, and a bypass passage, which allows coolant water discharged from the engine coolant water passage to return to the engine coolant water passage without passing through the radiator, a temperature sensor is provided on the radiator circulation passage in addition to a temperature sensor for detecting the temperature of the engine coolant water passage, so that an open failure of the thermostat valve can be detected by referring to a difference between the two temperature sensors. Specifically, in the case where the difference between the two temperature sensors is small even when a closing instruction is given to the thermostat valve, it is determined that the open failure occurs in the thermostat valve.
However, even when the thermostat valve is in a normal state (closed state), the leakage flow rate occurs in the radiator circulation passage during operation of the pump. Occurrence of this leakage flow rate causes the difference between the two temperature sensors to be small, so that there is a possibility that the thermostat valve is misdiagnosed as being in the open failure. Therefore, while it can be considered to take into consideration the leakage flow rate into the failure diagnosis for the thermostat valve, no observation is made as to this point for the failure detection system disclosed in Japanese Patent Laying-Open No. 2007-056722.